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A Novel Non-Platinum Group Electrocatalyst for PEM Fuel Cell Application

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A Novel Non-Platinum Group Electrocatalyst for PEM Fuel Cell Application ARTICLE IN PRESS international journal of hydrogen energy xxx (2010) 1e8 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he A novel non-platinum group electrocatalyst for PEM fuel cell application Jin Yong Kim*, Tak-Keun Oh, Yongsoon Shin, Jeff Bonnett, K. Scott Weil Pacific Northwest National Laboratory, Richland, WA 99352, USA article info abstract Article history: Precious-metal catalysts (predominantly Pt or Pt-based alloys supported on carbon) have Received 21 December 2009 traditionally been used to catalyze the electrode reactions in polymer electrolyte Received in revised form membrane (PEM) fuel cells. However as PEM fuel systems begin to approach commercial 19 April 2010 reality, there is an impending need to replace Pt with a lower cost alternative. The present Accepted 5 May 2010 study investigates the performance of a carbon-supported tantalum oxide material as Available online xxx a potential oxygen reduction reaction (ORR) catalyst for use on the cathode side of the PEM fuel cell membrane electrode assembly. Although bulk tantalum oxide tends to exhibit Keywords: poor electrochemical performance due to limited electrical conductivity, it displays a high Nanoscale tantaulum oxide oxygen reduction potential; one that is comparable to Pt. Analysis of the Pourbaix elec- PEM catalyst trochemical equilibrium database also indicates that tantalum oxide (Ta2O5) is chemically Oxygen reduction stable under the pH and applied potential conditions to which the cathode catalyst is typically exposed during stack operation. Nanoscale tantalum oxide catalysts were fabri- cated using two approaches, by reactive oxidation sputtering and by direct chemical synthesis, each carried out on a carbon support material. Nanoscale tantalum oxide particles measuring approximately 6 nm in size that were sputtered onto carbon paper exhibited a mass-specific current density as high as one-third that of Pt when measured at 0.6 V vs. NHE. However, because of the two-dimensional nature of this particle-on-paper structure, which limits the overall length of the triple-phase boundary junctions where the oxide, carbon paper, and aqueous electrolyte meet, the corresponding area-specific current density was quite low. The second synthesis approach yielded a more extended, three- dimensional structure via chemical deposition of nanoscale tantalum oxide particles on carbon powder. These catalysts exhibited a high ORR onset potential, comparable to that of Pt, and displayed a significant improvement in the area-specific current density. Overall, the highest mass-specific current density of the carbon-powder supported catalyst wase9% of that of Pt. Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. 1. Introduction reduction in Pt loading is still needed for large-scale auto- motive applications [1,2] and little success has been achieved While substantial progress has been made over the past in identifying suitable alternatives. At present over 30% of the decade in optimizing the anode/cathode structures and the stack cost is directly attributable to the cost of the platinum adjacent diffusion media and flow-field designs to reduce the catalysts employed in the electrodes [3], the majority of which amount of Pt needed in PEM fuel cell stacks, a five-fold is used in the cathode to catalyze the oxygen reduction * Correspondig author. Tel.: þ1 509 376 3372. E-mail address: [email protected] (J.Y. Kim). 0360-3199/$ e see front matter Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. doi:10.1016/j.ijhydene.2010.05.016 Please cite this article in press as: Jin Yong Kim, et al., A novel non-platinum group electrocatalyst for PEM fuel cell application, International Journal of Hydrogen Energy (2010), doi:10.1016/j.ijhydene.2010.05.016 ARTICLE IN PRESS 2 international journal of hydrogen energy xxx (2010) 1e8 reaction (ORR) shown in Equation (1) below. This reaction is overlooked for ORR catalysis: they are not electrically known to be the rate determining step in cell operation and conductive (at least in bulk form) at low-to-moderate the cause of reduced energy efficiency due to a large associ- temperatures [20]. This means that while there may be cata- ated over potential [4]: lytic sites on the surface of the oxide, the ORR will be kineti- þ cally hindered because electronic transport is limited. O þ 2H þ 2eÀ/H O (1) 2 2 An obvious means of rectifying this problem is to mix the Any viable replacement for Pt must possess the following oxide with a chemically stable electronic conductor such as characteristics: (1) good catalytic activity for the ORR, at least graphite. More specifically, the electrochemical performance within an order of magnitude of that exhibited by Pt; (2) good of tantalum oxide can be enhanced by constructing robust electrical conductivity; (3) long-term stability under nominal triple-phase boundaries (i.e. regions where oxide, graphite, cell operating conditions in the highly acidic aqueous envi- and aqueous electrolyte meet) that effectively serve as the ronment of the stack; and (4) low cost. Arguably, the most reaction sites for catalysis. The number and length of these restrictive of these requirements is the third. Over the past catalytic regions are maximized when the tantalum oxide is several decades, the search for non-precious-metal PEMFC finely dispersed as a bonded phase on a high surface area cathode catalysts has included numerous candidates, graphitic or carbon support material. To determine the including: transition metal carbides and nitrides (e.g. WC, viability of this concept, tantalum oxide was deposited in e e Mo2N), chalcogenides (e.g. Mo4.2Ru1.8Se8 and W Co Se), and discrete, nanoscale form on a typical PEM fuel cell grade N4-chleates (macrocycles such as iron phthalocyanine and carbon paper via reactive oxidation sputtering of a tantalum cobalt tetramethoxyphenylporphyrin) [5e10]. However, only target. Once feasibility was shown with sputtered samples, a few of these alternatives offer ORR onset potential and a more typical powder form of the catalyst was synthesized activities comparable to Pt, and those which come closest to Pt using a liquid-phase chemical synthesis approach that in terms of ORR catalytic activity fail to exhibit sufficient long- employs tantalum ethoxide as the tantalum source [21]. The term performance stability. Even the most promising and electrochemical properties of the sputtered and chemically thoroughly developed pyrolized metal porphyrins continue to synthesized carbon-supported tantalum oxide catalysts were display poor durability under practical fuel cell operating compared with a commercial Pt catalyst. conditions [11e13]. The present study investigates the performance of a carbon-supported nanoscale tantalum oxide catalyst as an 2. Experimental alternate ORR catalyst for PEM fuel cells. Although bulk tantalum oxide possesses poor electrochemical performance To demonstrate the concept of carbon-supported nanoscale due to its limited electrical conductivity, it exhibits a high tantalum oxide, tantalum oxide was deposited on 25.4 mm oxygen reduction potential (>0.95 V vs. RHE) that is compa- diameter discs of teflonized carbon paper (TGPH-090, E-TEK) rable to Pt. Stability diagrams listed in the Pourbaix electro- using reactive oxidation sputtering. In this process, the chemical equilibrium database [14] indicate that tantalum tantalum metal is DC magnetron sputtered in an atmosphere oxide (Ta2O5) is chemically stable under the pH and applied containing a partial pressure of oxygen that is sufficient to potential conditions to which the cathode catalyst is typically support in-situ oxidation during deposition. The carbon exposed during steady-state and transient fuel cell operation. substrates were mounted on a stationary water-cooled In addition, tantalum oxide is substantially lower in cost than backing plate in the sputtering chamber, which was subse- À Pt, by over 2 orders of magnitude. quently evacuated to 4 Â 10 8 Torr. The substrate surfaces À3 Ta2O5, as well as Nb2O5, received considerable attention were ion cleaned for 5 min in 0.8 Â 10 Torr argon. Ta sput- several decades ago as potent alloying additions that enhance tering was conducted under an atmosphere of 8 mTorr of Ar/ activity/selectivity and prolong life for various oxide catalysts 10% O2 at 0.3 A d.c., 290 V, which corresponds to an approxi- employed in the selective oxidation of hydrocarbons [15]. mate deposition rate of 0.4 mm/h. It was found that discrete Their use in electrocatalysts has been more limited, with nanoscale tantalum oxide particles were deposited on the primary interest focused on their corrosion stabilizing prop- carbon support when sputtering was carried over relatively erties [16]. For example, Ta2O5 additions have shown prelim- short time periods (e1e5 min). In this way, the average particle inary synergistic improvements in carbon-supported Pt size of the deposited tantalum oxide could be controlled in the catalysts (speculated to be due to the preferential adsorption range betweene6e80 nm by adjusting both the time period for of OH groups to the oxide vs. the Pt surface) [17]. Ribeiro and each sputtering “burst” and the total number of bursts, or Andrade [18] reported a significant increase in electrode life- aggregate sputtering time. time when adding Ta2O5 to TiO2/RuO2-based catalysts used in The powder form of the catalyst was prepared using oxygen evolution reactions (carried out under conditions a chemical synthesis approach that employed tantalum eth- 3 similar to the PEMFC: 0.5 mol/dm H2SO4 at 80 C and voltage oxide [Ta(OEt)5, 99.95%; Alfa Aesar] and nanoscale carbon sweeps of À0.2e1.2 V vs. RHE). Similarly, Bonakdarpour et al. powder (average particle sizee30 nm, Asbury Carbons, Grade #: [19] found that Ta markedly reduces the dissolution of Pt in 5345R) that was functionalized with eOH groups using sputtered Pt1ÀxTax alloy ORR catalysts by forming a thin Ta2O5 a previously reported procedure [21]. Nanoscale tantalum passivation layer. Unfortunately, the addition of Ta above w10 oxide particles form through direct reaction between the Ta at% also leads to a measurable decrease in ORR activity.
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